The present invention relates to printers, and more specifically, to apparatus and methods for printing by transferring ink from an ink film to a printing medium.
Thermal transfer printers are used for printing various documents with high resolution and full colors. When a thermal transfer printer prints documents of value, e.g., original tickets, gift certificates, postage stamps, and the like, there is a need to avoid counterfeiting of the documents. Security measures against counterfeiting include use of special ink (e.g., ultraviolet ink) and watermarked paper. For example, some hidden images are printed using special ink before other visible images are printed using regular color ink. Alternatively, images are printed on watermarked paper using regular black ink.
However, these security measures pose some problems. First, the cost of special ink or watermarked paper is high compared to regular ink or paper. Second, use of special ink would incur an additional mechanism and step because printing conditions (e.g., necessary temperature and pressure) of the special ink differs from those of regular ink. Finally, inventory control of special ink or watermarked paper against unauthorized use is usually not an easy task because the exact amount of remaining ink or paper is hard to manage.
In view of these and other issues, it would be highly desirable to have a technique enabling a thermal transfer printer to print images with some hidden security marks to avoid counterfeiting without using special ink or watermarked paper.
According to various embodiments of the present invention, a thermal transfer printer for transferring ink from an ink film to a printing medium has a print head, and a platen against which the print head is pressed during printing. The print head has a plurality of resistance heating elements operable to be individually energized with electrical drive pulses. Each of the resistance heating elements has a transfer surface which is substantially flat for transfer of the ink from the ink film to the printing medium. At least one of the transfer surfaces includes a concave portion which substantially avoids the transfer of the ink. A dot printed by a specific embodiment of the thermal transfer printer according to the present invention has an unprinted area. The unprinted area is not recognizable by the naked eye, but can be observable by using a magnifying device. Thus, the unprinted area in the dot printed by the thermal transfer printer functions as a watermark for determining authenticity of the printer.
In one specific embodiment, each of the plurality of resistance heating elements has the concave portion. In another specific embodiment, each of at least two of the transfer surfaces includes the concave portion, and each of the concave portions has a unique shape different from each other.